Tunnel importance, components, cross sections

Importance of tunnel

A tunnel is an underground passage excavated through a hill or mountain, or under water, without removing the overlying ground, to carry traffic.

Importance of road tunnels

• Shortens distance and travel time by avoiding steep climbs and long detours; especially useful in Nepal's hilly terrain.
• Lowers gradients, vehicle operating cost and fuel use.
• Avoids unstable slopes, landslides and snow- or avalanche-prone passes, and provides an all-weather connection.
• Reduces the environmental damage of large cuttings and hill-cut roads, and saves land in congested cities.
• Provides crossings under rivers, cities or sensitive areas without disturbing the surface; also useful for defence and utilities.
• Reduces accidents on hairpin bends.

Components and cross sections

          Crown / Arch (roof)
        ___________________
      /       Extrados       \
     |   ___ Intrados ___     |
 Haunch |  |  Springing line | |  Haunch
     |  | /               \ |  |
 Side   |/    CARRIAGEWAY    \|  Side
 wall   |  Footpath/Walkway   | wall
        |___Invert (floor)____|
        [Side drain]  [Side drain]

Components

• Portal: entrance/exit structure with wing walls and a retaining wall that protects the face from erosion and slides.
• Crown/roof (arch): top part of the lining; its inner surface is the intrados and its outer surface the extrados.
• Haunch: the part of the arch between the springing line and the crown.
• Springing line: the line where the arch meets the side walls.
• Side walls: vertical or curved walls that support the roof.
• Invert (floor/bottom arch): the floor, flat or curved, carrying the pavement; a curved invert resists upward pressure in squeezing ground.
• Lining: shotcrete, cast-in-place concrete or segmental lining supporting the ground.
• Drainage: side drains, sumps and cross-drains.
• Services: ventilation, lighting, emergency exits and cross-passages, signage, and fire safety systems.

Tunnel cross sections (shapes)

1. Circular: the best structural shape against all-round pressure; used in soft ground and in shield and TBM tunnels.
2. Horseshoe: arched roof with curved side walls and a flat or slightly curved invert; common for road and railway tunnels in rock.
3. D-shaped (segmental roof with vertical walls): flat floor, vertical side walls and an arched roof; used in hard rock where side pressure is small.
4. Egg-shaped/ovoid: for water or sewage tunnels.
5. Rectangular/box: used in cut-and-cover tunnels in cities.
6. Elliptical: used in soft ground where the vertical and lateral pressures are unequal.

The size follows the carriageway width (Nepal Road Standard 2070, plus shoulders and walkways), the vertical clearance (about 5.0 m) and the space needed for services.